Optical communication systems use pulses of light to transmit data. These optical communications systems use a number of components, such as lasers, the optical fibers, electroabsorption modulators (EAM""s), semiconductor optical amplifiers (SOA""s), and variable optical attenuators (VOA""s), which encode signals in the pulses of light, transmit the pulses and detect the optical signals. Optical monitors are an important element of integrated optical component devices and systems. Optical monitors typically produce a current, or voltage, proportional to the optical energy incident on them. This electrical signal may be used to provide both performance monitoring and an input signal for performance optimization circuitry. Performance monitoring and performance optimization are desirable to ensure reliable operation of optical communication systems.
Desirably, an optical monitor should not produce large currents or use excessive amounts of electrical power, nor should the optical monitor create undue loss of the optical signal being monitored through scattering or absorption.
Currently in the telecommunications industry optical monitoring is accomplished using a fiber fused coupler that bleeds off about 5% of the optical signal and sends it to a separate optical detector. These couplers are larger and more expensive than may be desirable and also require fiber splicing in the system, which may lead to coupling losses. Additionally, some couplers, or other components, may be damaged during the delicate fiber splicing operations.
When deploying an optical transmitter into an optical communication network, it is also desirable to have algorithms that continually maintain and optimize the optical performance. One form of optical transmitter whose operation may be desirably improved by optical monitoring is an EAM. A valuable control algorithm for an EAM is to maintain a constant optical absorption across the modulator for the modulated signal. Maintaining a constant optical absorption across the modulator may ensure that the device is operating under the desired bias conditions. The ratio of this optical absorption to the transmitted signal is known as the extinction ratio of the modulator. Maintaining a constant extinction ratio may allow consistent optical performance, even with variations of wavelength and age-induced changes or environmentally-induced changes, such as temperature changes, in the performance of components of the optical communications system, including the EAM. Consistent optical performance for various input wavelengths may also be achieved.
One embodiment of the present invention is a monolithic stabilized electroabsorption modulator (EAM). An exemplary monolithic stabilized EAM includes: a substrate; a waveguide layer; a semiconductor layer. The waveguide layer is coupled to the top surface of the substrate and includes an electroabsorption medium, which interacts with light of the predetermined wavelength, and is responsive to an electric signal. The electric signal is applied between the substrate and the semiconductor layer. The waveguide layer includes an output optical tap section and a modulation section which is adjacent to the output optical tap section. These sections each include a portion of the electroabsorption medium. An output tap electrode and a modulator electrode may be coupled to the semiconductor layer opposite the output optical tap section and modulation section of the waveguide layer, respectively.
A further embodiment of the present invention is a method of manufacturing an exemplary monolithic stabilized EAM which includes a substrate; a waveguide layer with an output optical tap section and an electroabsorption section arranged along a longitudinal axis; and a semiconductor layer. The method begins with forming a waveguide layer on the top surface of the substrate. The waveguide layer has a different index of refraction than the substrate. The waveguide layer includes an electroabsorption portion which is adjacent to the output optical tap portion. The electroabsorption portion of the waveguide layer may also include a plurality of sub-layers of waveguide material forming a quantum well structure. Next, the semiconductor layer, which has an index of refraction different from the index of refraction of the waveguide, is formed on the waveguide layer. The waveguide layer and the semiconductor layer are then defined and etched to form a mesa structure. A base electrical contact is deposited on the substrate and a modulator electrical contact and output optical tap electrical contact are deposited on the semiconductor layer.
Still another embodiment of the present invention is a method of stabilizing the extinction ratio of a monolithic stabilized EAM, which includes an input optical tap, an EAM, and an output optical tap. The method begins by supplying a bias voltage to the input optical tap, the EAM, and the output optical tap. Next, the input tap current of the input optical tap and the output tap current of the output optical tap are detected. The extinction ratio of the EAM is then calculated based on these tap currents. The bias voltage is varied based on the calculated extinction ratio to control the extinction ratio at a predetermined level.
An additional embodiment of the present invention is a method of stabilizing the extinction ratio of a monolithic stabilized EAM, which includes an EAM and an output optical tap. The method begins by supplying an input optical signal to the monolithic stabilized EAM and supplying a bias voltage to the EAM and the output optical tap. A periodic variation in the input optical signal is generated, which has a variation amplitude and a variation frequency. Next the output tap current of the output optical tap is detected, using synchronous detection at the variation frequency. The extinction ratio of the EAM is then calculated based on the output tap current. The bias voltage is varied based on the calculated extinction ratio to control the extinction ratio at a predetermined level.
A still further embodiment of the present invention is a method of stabilizing the extinction ratio of a monolithic stabilized EAM, which includes a temperature control element, a temperature sensor, an EAM, and an output optical tap. The method begins by supplying a bias voltage to the EAM and the output optical tap and supplying a temperature control voltage to the temperature control element. The temperature of monolithic stabilized EAM is measured using the temperature sensor and the temperature control voltage is varied based on the measured temperature to regulate the temperature of monolithic stabilized EAM to an operating temperature. Next the optical tap current of the output optical tap is detected. The extinction ratio of the EAM is then calculated based on the output tap current. The operating temperature is varied based on the calculated extinction ratio to control the extinction ratio at a predetermined level. Additionally, the bias voltage may be varied based on the calculated extinction ratio to provide additional control of the extinction ratio at a predetermined level.
Another embodiment of the present invention is a tunable EAM system, which includes a monolithic stabilized EAM, a tap current monitor, extinction ratio calculation circuitry, a bias controller, and a temperature controller. The monolithic stabilized EAM includes a substrate a waveguide layer coupled to the substrate, a semiconductor layer coupled to the waveguide layer, a temperature control element coupled to the substrate, and a temperature sensor coupled to the substrate. The waveguide layer includes an output optical tap section which is defined by an output tap electrode and a modulation section which is defined by a modulation electrode. The tap current monitor is electrically coupled to the output tap electrode of the monolithic stabilized EAM to measure the tap current. The extinction ratio calculation circuitry is coupled to the tap current monitor and is adapted to determine a desired bias voltage of the optical tap section and the modulation section of the monolithic stabilized EAM and a desired operating temperature of the monolithic stabilized EAM based on the output tap current. The bias controller is coupled to the extinction ratio calculation circuitry and electrically coupled to the output tap electrode and the modulator electrode of the monolithic stabilized EAM to supply the bias voltage determined by the extinction ratio calculation circuitry to the output optical tap section and the modulation section of the monolithic stabilized EAM. The temperature controller is coupled to the extinction ratio calculation circuitry, as well as the temperature control element and the temperature sensor of the monolithic stabilized EAM to control the temperature of the monolithic stabilized EAM at the operating temperature determined by the extinction ratio calculation circuitry.